The Cos Cob Power Plant was conceived in 1903 as a result of the New York State legislature’s law that prohibited all steam locomotives from entering New York City after 1908 due to train wrecks in the Park Avenue Tunnel caused by low visibility from locomotive smoke and steam. The New York Central Railroad, which operated all railroad traffic between Woodlawn, New York, into New York City, decided to install a low-voltage direct current third-rail electrical system; thus after 1908 all locomotives coming into New York City's Grand Central Terminal (then under construction), would be forced to operate off third- rail.
Although the New York, New Haven & Hartford Railroad, better known as the New Haven Railroad, operated considerable low- voltage direct current trackage at this time, this system was considered inadequate for the operation of heavy trains over long distances at high speeds. The New Haven Railroad’s engineers, working with Westinghouse Electric and Manufacturing Company of Pennsylvania, decided to install high-voltage 11,000 volt overhead wires on the tracks from Woodlawn, New York, into Connecticut. These wires would be powered from the railroad's own power station, using single-phase alternating current (ac) electricity. This was done at a time when low-voltage direct current electrification was the rail standard of the day.
Construction of the power plant began in 1905 in Cos Cob, Connecticut, located in the town of Greenwich on the Mianus River. On 24 July 1907, the first New Haven Railroad electrified passenger train traveled from Grand Central Terminal to New Rochelle, New York. The initial electrification covered all four tracks to Stamford, Connecticut, and in 1913-1914 was extended to New Haven, Connecticut.
The New York Central Railroad's third-rail system from Woodlawn, New York, into Grand Central Terminal required the New Haven Railroad's locomotives to operate both off this system and on their own high-voltage overhead wire. This meant that these locomotives and multiple unit cars had to change over between these two systems while moving at track speed. With so much traffic it was considered unworkable to stop every train for the changeover.
The initial overhead catenary construction from Woodlawn to Stamford utilized two parallel messenger wires supporting a single trolley wire by vertical hangers, resulting in a triangular construction. The rigidity of this arrangement was found to be undesireable, so the 1913-1914 electrification extension to New Haven utilized a single messenger wire, as did other extensions. The system reached 673 track miles at its maximum.
The Cos Cob Power Plant was the first power plant built exclusively for a railroad and is considered an engineering achievement for its use of high-voltage alternating current (ac) for railroad electrification. The plant served the New Haven Railroad until the railroad's demise in 1969, and then served Penn Central, Conrail and Metro-North until it was closed in 1986. The Cos Cob Power Plant received landmark status from the American Society of Mechanical Engineers/Institute of Electrical and Electronics Engineers in 1982.